1. Field of the Invention
The present invention relates to a combustion control device for an engine which performs premixed charge compression ignition (PCCI) combustion.
2. Related Background Art
Known as an example of combustion control device for engines which perform premixed charge compression ignition combustion is one disclosed in Japanese Patent Application Laid-Open No. 2003-83119 (Patent Literature 1). The combustion control device disclosed in Patent Literature 1 has a conventional injection mode and a premixed compression injection mode as fuel injection modes. The conventional injection mode, which is used when an engine is under high load, injects a fuel from a fuel injection nozzle at a timing near a compression top dead center. The premixed compression injection mode, which is used when the engine is under low or medium load, injects the fuel from the fuel injection nozzle at a timing ranging from 50° to 20° before the compression top dead center. The injection angle at this time is set to such a given angle that the fuel reaches the inner wall of a cavity on the radially outer side from the lowermost position of the cavity in the conventional injection mode and at least the inner wall of the cavity in the premixed compression injection mode.
In the premixed charge compression ignition combustion of the prior art mentioned above, the fuel is injected to the inner wall of the cavity, whereby unburned HC is inhibited from being generated by the fuel attaching to the bore wall of the cylinder. However, there have been no proposals for measures against the production of unburned HC by premixed charge compression ignition combustion when the engine is under low load in particular. More specifically in this regard, during the low load operation of the engine, the fuel is injected by such a small amount as to become so thin when spread throughout the cavity that a part thereof fails to burn. Here, the phenomenon that the fuel diffuses thinly to such a level that it partly fails to burn will be referred to as excessive diffusion of the fuel. Parts hindered by the excessive diffusion from burning normally, if any, will generate unburned HC/CO. As a measure against it, first the fuel may be injected in the conventional injection mode when the engine is under low load as with high load. When injected near the compression top dead center in the conventional injection mode, the fuel is ignited sequentially, whereby the excessive diffusion is avoided. However, it cannot yield effects achieved by premixed charge compression ignition combustion such as less NOx.